I disable expa sion but also biters evolution. Without expansion and evolution biters are just the right amount of pressure for me for casual play without completely losing having to climb half of the game tech tree
For some reason I never had a problem with the biters on the default difficulty. I got the sense they were there as another ball to juggle rather than a primary antagonist. Simply building turrets around structures was enough to fend them off in the early game until I could proactively destroy their nests in the pollution zone. After I started doing that, they never bothered me again.
It's not about the challenge tbh i play rail worlds because I like trains and while it evens out long term the start is slower and I dont feel like having time based evolution is a fair or interesting. I do leave pollution based evolution tho. That feed into base design choices